Theoretical Study on the Raman Effect Due to Magnons in Two-Dimensional Magnets.

Raman spectroscopy is one of the most useful experimental tools for studying elementary excitations in two-dimensional (2D) materials. The Raman scattering due to phonons was widely employed for detecting structural evolutions, especially those caused by magnetic phase transitions in 2D magnets. A first-principles theory of the Raman scattering effect caused by magnons is still lacking. We theoretically study the magnon Raman effect in 2D magnet CrI. We propose a first-principles method and have calculated the intensity of circularly polarized Raman signals due to different magnon modes in the CrI monolayer and bilayers. The calculated Raman intensities due to magnons in the CrI monolayer and the rhombohedral bilayer are consistent with the selection rule deduced from the magnon pseudoangular moment and the parity of magnon modes. We also find that the selection rule is violated in the symmetry-broken monoclinic bilayer due to interlayer coupling.